Simplify kotlin code and improve code readability (#1198)

* Add kotlin code block for chapter_hashing

* Add kotlin code block for chapter_heap.

* Add kotlin code block for chapter_stack_and_queue and chapter_tree

* fix indentation

* Update binary_tree.md

* style(kotlin): simplify code and improve readability.

* simplify kt code for chapter_computational_complexity.

* style(kotlin): replace ArrayList with MutableList.

* Update subset_sum_i.kt

Use kotlin api instead of java.

* Update subset_sum_ii.kt

use kotlin api instead of java

* style(kotlin): replace ArrayList with mutablelist.

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Co-authored-by: Yudong Jin <krahets@163.com>

codes/kotlin/chapter_array_and_linkedlist/array.kt

@@ -55,7 +55,7 @@ fun traverse(nums: IntArray) {
         count += nums[i]
     }
     // 直接遍历数组元素
-    for (j: Int in nums) {
+    for (j in nums) {
         count += j
     }
 }
@@ -63,7 +63,8 @@ fun traverse(nums: IntArray) {
 /* 在数组中查找指定元素 */
 fun find(nums: IntArray, target: Int): Int {
     for (i in nums.indices) {
-        if (nums[i] == target) return i
+        if (nums[i] == target)
+            return i
     }
     return -1
 }